Micromechanical measuring elements are used in sensors for different areas of application. For example, acceleration sensors, yaw rate sensors, or pressure sensors may be micromechanically constructed. Such a measuring element includes both mechanical and electrical structures, which may have similar orders of magnitude and form an integrated micro-electromechanical system (MEMS) with each other. Such micro-electromechanical systems also occasionally include an actuator for parts of the mechanical structures, for example in yaw rate sensors in which a force acting upon a movable mass is to be determined. A micro-electromechanical resonator may also include such a drive.
In one form of micro-electromechanical measuring elements, a relative deflection between mechanical elements is analyzed, at least one of the mechanical elements at the same time also being part of an electronic element, which provides an output signal correlating with the mechanical deflection. Such a deflection may have the order of magnitude of the structures from which the measuring element is made. The deflection may be determined, for example, by determining a deflection-dependent capacitance.
For example, in the “moving gate” technique, a flat gate electrode of a field-effect transistor (FET) may be mounted parallel to a channel of the FET and deflected with respect to the channel. The channel is delimited by a source and a drain terminal of the field-effect transistor. A voltage is applied between the source and the drain terminals, and the gate electrode is electrically connected to the drain terminal, so that the field-effect transistor is operated as a current controller. A current flow through the field-effect transistor then changes as a function of the electric field, which is established due to the voltage between the gate electrode and the channel. If the configuration of gate electrode and channel is modified, the current flow through the field-effect transistor is ultimately also modified. Capacitive measurement is suitable for MEMS measuring sensors, since in this case there is virtually no reaction from the (deflection-dependent) current flow on the deflection of the electrode. The gate electrode and the channel are usually shaped in such a way that a region in which the two overlap is linearly changeable to a deflection of the gate electrode. The deflection modifies the size of the overlapping region, so that the electric field between gate electrode and channels, which controls the current flow through the field-effect transistor, is modified. The deflection of the gate electrode may thus be determined with the aid of the current flow through the field-effect transistor.
U.S. Pat. No. 6,220,096 B1 discusses a micro-electromechanical acceleration sensor which uses moving gate field-effect transistors (MG-FET) of the type described above in a differential circuitry for optimizing a useful signal of the sensor.
Measuring systems that process a sensor signal should usually provide an output signal which may have a linear relationship with a quantity to be measured. For this reason, usually all elements of the system are laid out to have linear characteristic curves, so that the required linear relationship exists over the entire system. In particular in the case of systems including a large number of complex processing elements, it is, however, difficult to ensure the linearity of each individual element in the required quality.